A grant is shared among three heads. The third head is given 20% of what the first two heads get together. What per cent of the grant does the third head get?

Let the first two heads together get $F$. Then the third gets $0.20F$ and the grant is $F+0.20F=1.20F$, so the third head's share is $0.20F/1.20F=1/6=16$ 2/3%. Numbers: if the first two get ₹100, the third gets ₹20 and the grant is ₹120. 20% is the trap - that rate is taken of the OTHER TWO heads, not of the grant, and a part can never be 20% of the whole and 20% of the rest at the same time. 25% is 20/80, the reading you get if the third head is set at 20% of the grant. 80% is the first two heads' share under that same wrong reading.